"If aliens visit us, the outcome would be much as when Columbus landed in America, which didn't turn out well for the Native Americans," he said. "We only have to look at ourselves to see how intelligent life might develop into something we wouldn't want to meet."

In the new program, "Into the Universe with Stephen Hawking," he speculated that aliens' capabilities "would be only limited by how much power they could harness and control, and that could be far more than we might first imagine."

He said it might even be possible for aliens to harvest the energy from an entire star.

"Such advanced aliens would perhaps become nomads, looking to conquer and colonize whatever planets they can reach," Hawking said.

Stephen Hawking Warns About Alien Life

Humans Only Recently 'Tapped Into Our Cosmic Neighborhood'

But don't start worrying quite yet. It's unlikely that those traveling troublemakers will visit us anytime soon, said space watchers.

Jill Tarter, director of the Center for SETI (Search for Extraterrestrial Life) Research at the SETI Institute, said that her center uses radio telescopes and optical telescopes to listen for signals of technology from extraterrestrial life. So far, after more than 40 years, there has not been a peep.

She said SETI's technology is advanced enough that it can detect signals from up to 1,000 light-years away. There are about one million stars in that zone. A signal could have been sent 1,000 years ago, before that civilization had any knowledge of Earth.

Scientists Search for Extraterrestrial Life

"It's quite reasonable that we might be on someone's transmission list," she said.

She emphasized, however, that though it's an effort worth considering, SETI doesn't actively transmit messages to space. So far, it has only listened.

"The question of whether or not we should transmit is a question that deserves a global conversation, and we're trying to figure out how to have that," Tarter said.

Ian O'Neill, space producer for Discovery News, an ABC News partner, said that humans didn't start leaking transmissions into space until the first radio broadcasts about 100 years ago. Given that our galaxy alone is 100,000 light years across, relatively speaking, he said, those signals haven't traveled too far.

"We've only tapped into our cosmic neighborhood recently," he said."That time scale is huge."

He also said that though scientists believe that life exists across the universe, there's no actual evidence of it yet. It could be hundreds, if not thousands, of years, he said, before human messages get an extraterrestrial response.

And if aliens do visit Earth, who knows what they would be like, he said.